1. Field of the Invention
The invention relates to polycrystalline silicon for semiconductor and photovoltaic applications, and to a process for production thereof.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) serves as a starting material for producing monocrystalline silicon for semiconductors by the Czochralski (CZ) or float zone (FZ) process, and for producing mono- or polycrystalline silicon by different pulling and casting processes for production of solar cells for photovoltaics. It is generally produced by means of the Siemens process. In this process, thin filament rods of silicon are heated by direct passage of current in a bell-shaped reactor (“Siemens reactor”), and a reaction gas comprising a silicon-containing component and hydrogen is introduced. The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n n=0, 1, 2, 3; X═Cl, Br, I). It is preferably a chlorosilane (X═Cl), more preferably trichlorosilane (n=1). SiH4 or SiHCl3 is predominantly used in a mixture with hydrogen. The filament rods are inserted vertically into electrodes at the reactor base, through which they are attached to the power supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the rod diameter grows with time.
The process is controlled through the setting of the rod temperature and reaction gas flow and composition. The rod temperature is measured with radiation pyrometers on the surfaces of the vertical rods. The rod temperature is set by controlling or regulating the electrical power, either at a fixed level or as a function of the rod diameter. The reaction gas rate is set as a function of the rod diameter. The deposition conditions are selected such that the rod diameter grows in the form of homogeneous and hole-free layers, i.e. the silicon rods thus obtained are very substantially free of cracks, pores, seams, fissures, etc., and are thus homogeneous, dense and solid. Such a material and the processing thereof are described, for example, in U.S. Pat. No. 63,50,313B2. The apparent density of such compact polysilicon corresponds to the true density of polysilicon and is 2.329 g/cm3.
The polysilicon rods thus obtained have to be processed to lumps and chips if they are not used for the production of single crystals by the FZ process. To this end, the rods are comminuted with tools such as hammers, crushers or mills and then classified by size. The smaller the fragment size and the higher the strength of the polysilicon rods, the greater the contamination of the polysilicon by the tools.
For the production of monocrystalline and polycrystalline silicon, crucibles are filled with fragments of different size. For the first filling, the aim is a maximum fill level of the crucibles. For this purpose, silicon pieces of very different size and weight, i.e. sawn rod pieces, coarse lumps, small chips and fine material, have to be mixed. The size of the silicon pieces ranges from <1 mm up to pieces of 150 mm and more; the shape of the pieces must not deviate too greatly from the spherical form.
For the multiple refilling of the crucibles, only fine, free-flowing, i.e. substantially spherical, fragments are suitable, since the material has to be conveyed through tubes and fittings into the crucible and must neither damage the crucible nor excessively disturb the silicon melt.
The yield of the crucible pulling operations is limited by the amount of impurities which become enriched in the silicon melt, which are in turn introduced predominantly through the fine silicon fragments.
Since the crystal pulling process is sensitive to the size distribution and form of the polysilicon used, a ratio of width to length (W/L) of the silicon fragments of 0.7 to 1.0 and a sphericity of the silicon fragments of 0.7 to 1.0 has become established as a de facto standard for use in crystal pulling processes. An example of the different fragment size ranges customary on the global market for controlled crucible setup with maximum crucible fill level can be found, for example, on the following web page of Wacker Chemie AG, on which fragment size fractions with a maximum length of the silicon fragments of 5-45 mm, 20-65 mm, 20-150 mm are advertised:    (http://www.wacker.com/internet/webcache/en_US/PTM/Poly silicon/PolyChunks/Polysilicon_chunks_etched.pdf).
The length L denotes the maximum dimension of a particle; the width W is the dimension at right angles to the maximum dimension. The sphericity is defined as the diameter of the circle with the same projection area as a particle divided by the diameter of a circle which encloses the particle projection (definition according to Wadell for two-dimensional analysis areas).
US 2003/0150378 A2 discloses “teardrop poly” and a method for producing it. In this method, a compact hole-free high-purity polysilicon rod (“stem”) is deposited from monosilane SiH4 by means of the Siemens process at 850° C. and a silane concentration of 1.14 mol % up to a silicon rod diameter of 45 mm. Subsequently, the rod surface temperature is suddenly increased from 850 to 988° C. and the silane concentration is suddenly reduced from 1.14 to 0.15 mol %. This parameter jump suddenly alters the growth of the silicon crystals on the silicon rod, and needles, known as dendrites, grow from the rod surface. Subsequently, the rod surface temperature is lowered continuously, such that the further growth of the needles to form large “teardrops” is continued until the end of the deposition. The “teardrops” are droplet-shaped structures which are connected to the stem only by their narrow ends and are not fused to one another. In the production of silicon fragments, this enables the teardrops to be broken easily off the “stem”. This polysilicon and the method for producing it have a series of disadvantages:
The polysilicon rod is very inhomogeneous. It consists of a compact, crack- and fissure-free and hence solid “stem”, and the “teardrops” which are separated from one another by cavities and are not fused to one another. Once the “teardrops” have been removed, the stem has to be processed further separately. This means additional work in the form of a two-stage process, possibly even with intermediate storage of material. The relative proportions by mass of stem and teardrops are determined by the separation process. In contrast to a homogeneous material, the size distribution of the comminuted material can therefore no longer be selected freely. Owing to the lack of connection between the teardrops, the current flows exclusively through the stem. The diameter thereof therefore cannot be selected at as low a level as desired, since it would otherwise melt. Since the currents required increase with rising diameter in the deposition, this means that the diameter of the stem must also increase. Thus, only a proportion of the silicon deposited, which decreases with rising rod diameter, is available as teardrops.
The form of the teardrops differs significantly from the fragments obtained from compact silicon rods in terms of size distribution, sphericity and W/L ratio. This material is therefore not usable for production of mono- or polycrystalline silicon without adjustment of the silicon pulling processes.